Hitherto, thin-film transistors (TFTs) formed by using amorphous silicon, or polycrystalline silicon have been widely used as switching elements of a liquid crystal display device, an organic electroluminescence (EL) display device and the like. However, chemical vapor deposition (CVD) apparatuses used for producing such TFTs using silicon are expensive. Thus, the production of large TFT elements results in an increase in the production cost. In addition, since silicon materials are deposited at high temperatures, in view of a problem of heat resistance, these materials cannot be applied to plastic substrates, which are likely candidates for substrates of flexible displays in the future. To solve this problem, organic TFTs have been proposed in which, instead of a silicon semiconductor, an organic semiconductor is used in a channel semiconductor layer.
By preparing a solution of an organic semiconductor, a film of the organic semiconductor can be formed by printing at a low temperature. Accordingly, organic semiconductors do not require large-scale manufacturing facilities, and can also be applied to plastics, which have poor heat resistance. Thus, organic semiconductors are expected to take the lead in flexible displays. On the other hand, organic semiconductors have a problem in practical application in that carrier mobilities of organic semiconductors are lower than those of silicon semiconductors, resulting in a decreased response speed of TFTs. Recently however, organic semiconductors having mobilities equivalent to that of amorphous silicon have been developed.
For example, PTL 1 describes a compound having a 2,7-substituted[1]benzothieno[3,2-b][1]benzothiophene skeleton (hereinafter, [1]benzothieno[3,2-b][1]benzothiophene is abbreviated as “BTBT”). PTL 1 describes compounds having, as the substituents, substituents selected from halogens, C1-C18 alkyls, C1-C18 alkyls having halogens, C1-C18 alkyloxys, C1-C18 alkylthios, aryls, and aryls having at least one selected from halogens, C1-C18 alkyls, C1-C18 alkyls having halogens, C1-C18 alkyloxys, and C1-C18 alkylthios. It is described that the mobility (cm2/Vs) of these compounds is 0.17 to 0.31 cm2/Vs.
PTL 2 describes a compound having a 2,7-substituted BTBT skeleton. PTL 2 describes compounds having, as the substituents, substituents selected from a hydrogen atom and halogeno-substituted C1-C36 aliphatic hydrocarbon groups. It is described that the mobility (cm2/Vs) of these compounds is 0.12 to 4.5 cm2/Vs.
PTL 3 describes a compound having a 2,7-substituted BTBT skeleton having a chalcogenophene ring. It has been reported that adhesiveness between a semiconductor thin film and an electrode and thin-film stability in the air were improved by introducing a chalcogenophene ring, and that the mobility was 0.08 to 0.22 cm2/Vs.
Furthermore, PTL 4 to PTL 6 describe compounds having a benzothienothiophene skeleton having an acetylene structure. It is described that these compounds are useful as a precursor of an organic semiconductor material and a precursor of an organic semiconductor polymer.
PTL 7 describes compounds that have various BTBT Skeletons and exhibit a high-order liquid crystal phase. However, derivatives according to the present invention are not known.
PTL 8 describes a compound for an organic thin-film transistor, the compound having a structure including an aromatic heterocyclic ring as a skeleton. However, derivatives according to the present invention are not described. According to the description of Examples, the order of the field-effect mobility is about 10−1 to 10−2 (cm2/Vs).
PTL 9 describes that a specific organic compound having, at the center, an aromatic hydrocarbon group or an aromatic heterocyclic group and an acetylene structure can be used in an organic thin-film transistor. However, it is not described whether or not the compound exhibits a high-order liquid crystal phase, which is an object of the present invention.
As described above, many organic semiconductor materials having an improved mobility have been reported. However, in the case where an organic semiconductor thin film is formed by printing, it is difficult to obtain an organic semiconductor film having a high mobility because the organic semiconductor molecules are arranged randomly at the time of printing, and thus a flow path through which carriers flow is not formed. Accordingly, in this application, an organic semiconductor material that provides a film in which organic semiconductor molecules are easily oriented in one direction at the time of film formation by printing and through which carriers easily flow is required.